Which of these numbers is prime? ${9,\ 10,\ 27,\ 39,\ 89}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 10 are 1, 2, 5, and 10. The factors of 27 are 1, 3, 9, and 27. The factors of 39 are 1, 3, 13, and 39. The factors of 89 are 1 and 89. Thus, 89 is a prime number.